In 2005, the commercial polybrominated diphenyl ether (PBDE) mixture known as PentaBDE - a widely used brominated flame retardant (FR) - was voluntarily phased out in the United States due to concerns about persistence, bioaccumulation, and toxicity. Due to increased use as PentaBDE replacements, organophosphate-based FRs (OPFRs) have now been detected at concentrations comparable to and, in some cases, higher than total PBDE concentrations in household dust, suggesting that chronic human exposure is common within the US. Using zebrafish as a model, our long-term goal is to identify xenobiotic-mediated pathways that contribute to adverse outcomes during early vertebrate development. As a step toward this long-term goal, the objective of this application is to uncover mechanisms of developmental toxicity for two high-production volume OPFRs commonly detected within indoor environments. Our central hypothesis is that two major classes of OPFRs - chlorinated phosphate esters (CPEs) and aryl phosphate esters (APEs) - widely used as FRs and plasticizers exhibit distinct modes of action during embryogenesis. Specifically, our working hypotheses are that (1) the CPE tris(1,3-dichloro-2-propyl) phosphate (TDCPP) inhibits DNA methyltransferase (DNMT) activity and, as a result, delays zygotic genome methylation during early embryogenesis and (2) the APE triphenyl phosphate (TPP) prevents normal cardiac looping through aberrant activation of a retinoic acid receptor (RAR)-dependent pathway. These hypotheses were formulated based on preliminary data from the applicant's laboratory. The central hypothesis will be tested by pursuing two specific aims: 1) Identify the epigenetic mechanism responsible for TDCPP-induced delays in zygotic genome methylation during cleavage; and 2) Identify the role of aberrant RAR activation in TPP-induced looping impairments during heart morphogenesis. For the first aim, we will rely on a combination of early embryonic exposures, fluorometric DMNT activity assays, and methylated DNA affinity capture coupled with high- throughput next-generation sequencing (MethylCap-seq). For the second aim, we will rely on a combination of high-content screening (HCS) assays, reverse genetics, real-time PCR, and in vitro human RAR reporter assays. The proposed research is innovative because, for the first time, we will leverage the power of (1) MethylCap-seq to reveal chemically-induced effects on the zebrafish embryonic methylome and (2) our existing 384-well-based HCS assays coupled with reverse genetics to identify the role of RAR activation in chemically-induced effects on cardiovascular development within zebrafish embryos. This contribution is significant because it (1) begins to address key uncertainties about mechanisms of developmental OPFR toxicity; (2) helps prioritize targeted, mechanism-focused evaluations using human cell line-based assays and prenatal developmental toxicity studies within rodents; and (3) raises questions about the potential health risks of two widely used OPFRs to developing human embryos resulting from chronic and ubiquitous exposure.